He didn't chose to be. He didn't have a choice. He never had actually.

Oh btw, Amon occasionally let Nain out of his belly during those 400 years. Nain first tried to adapt to the new environment, tried to rule the people with wisdom, but in the end he would go on depression and back to Amon belly to sleep. Until that hight priest ancestor offered Amon a alternative for Nain depression ( that is erasing his memory by drugs)

Nain and Amon met 400 years ago. Amon was sitting on a rock on a river, waiting for his destined mate. Nain saw him, and fell for his beauty and mysterious look, revered him like a g**, bc Amon was literally just sitting there like a statue not moving an inch for days/months. 🗿
They talked (mostly Nain), Nain gave Amon flowers and food, and some time later they promised to marry each other. But only after Nain coming of age.
Well, he grow up, and tragedy hits him. Nain was killed by lunatics, and to revive Nain, Amon ate his flash and restored Nain body inside his belly. (That where Nain nightmare come from).
Well Nain passed most of the 400 years sleeping inside Amon, while Amon made a kingdom for Nain just bc he said he wanted to be rich and have lot of golds.